The goal of this proposal is to develop and clinically test a real-time optical imaging modality based on confocal reflectance microscopy, with enhancement of nuclear-to-dermal contrast using acetic acid and crossed polarization, to intra-operatively examine basal cell cancers to guide Mohs micrographic surgery. Basal cell cancers (BCCs) are among the fastest growing cancers (>1.2 million new USA cases/year); because they occur most frequently on older people and in high risk anatomical sites (near or on the eyes, mouth, nose, ears), precise microsurgery is necessary with minimum damage to the surrounding normal tissue. Mohs surgery is time-consuming (one to several hours) and tedious because several (2-20) excisions must be made, and frozen histology sections carefully prepared and examined for each. Realtime confocal reflectance imaging may make Mohs surgery significantly more efficient by enabling direct intraoperative examination for cancer nests and cancer-to-normal tissue margins on the patient; this will reduce the number of excisions and avoid frozen histology. Both the patient and surgeon will save several hours per procedure in the operating room. Although proposed for Mohs surgery of BCCs, this real-time intra-operative imaging modality may, in fact, prove useful in a variety of other microsurgical settings. The research will be a collaboration between Northeastern University (Boston) and Memorial Sloan-Kettering Cancer Center (New York). This project will be implemented in two phases: (1) an ex vivo study using freshly excised skin specimens from Mohs surgeries, to (a) design a confocal microscope that mimics the Mohs surgeon's examination of frozen histology: rapid low-resolution examination of BCC nests in wide fields-of-view followed by high-resolution inspection of nuclear morphology in small fields-of-view, and develop (b) optimum contrast enhancement methods using topical acetic acid (induces condensation of chromatin that increases light backscatter from nuclei) and optical crossed polarization (suppresses light backscatter from the surrounding normal dermis), (c) optimum hemostasis methods using either topical aluminum chloride or electrocautery, (d) image understanding by detailed correlation of images to histology; and (2) an intraoperative study, involving (a) design of a specialized confocal articulated telescope, to enable (b) imaging of BCCs on patients during surgery, to evaluate the optimum contrast enhancement and hemostasis methods, correlate confocal images to histology and quantitatively determine diagnostic/screening accuracy. The overall goal is to develop an optical (confocal) intra-operative imaging instrument for Mohs and other modes of microsurgery.